Prior art methods and devices have been proposed for detecting the in-focus condition of an image forming optical system by evaluating the sharpness of an image formed by said optical system. In one class of systems, as shown in U.S. Pat. No. 4,341,953 issued Jul. 27, 1982 to Sakai et al, photo-sensitive elements having output characteristic which vary in accordance with image sharpness are positioned on opposite sides and at substantially equivalent distances from the plane of best focus. In one arrangement as shown in FIG. 8 herein, an image formed by a lens 10 is split by beam splitters 12 and 12' and focused on a plurality of image sensors 14, 14' and 14". Image sensor 14 is located at the plane of best focus, image sensor 14' is located in front of the plane of best focus and image sensor 14" is located behind the plane of best focus. The signals produced by the image sensors are employed to determine the plane of best focus. The problem associated with using beam splitters is that the amount of light reaching the individual image sensors is reduced, thereby reducing the signal to noise ratio of the focus sensor and making this approach less effective in low light level or short exposure situations. Where the image is moving, the motion of the image compels short exposure times to obviate image smear.
In another arrangement, as shown in FIG. 9 herein, the lens 10 focuses an image onto a plurality of image sensors 14, 14' and 14" located in a plane that is tilted with respect to the optical axis of the lens. This arrangement avoids the light loss associated with beam splitters but has the disadvantage that different image sensors sense different portions of the image. Where the different portions of the image contain different amounts of image detail, the sensor can give a false focus reading, making the sensor fail to perform properly.